In 2005, he appeared to endorse a Pfizer-backed report offering NHS patients easier access to private-sector healthcare, writing that “we believe that choice is a good thing” in a press release.

Watch | Owen Smith: Labour can't just be a protest party

01:35

Supporters of the Labour leadership candidate reacted with fury over the critical quotes from Corbyn allies.

One Labour MP accused allies of “smearing” Mr Smith while another supporter insisted he would run a “positively and comradely campaign” and urge the incumbent to do the same.

Jeremy won't be getting into personal abuse, as he never has doneSenior Labour source

Corbyn associates were quoted in The Times calling Mr Smith a “Blairite-lite candidate” and a “TV-savvy turncoat” despite the Labour leader’s promise to avoid personal abuse during the contest.

A senior Labour source denied Mr Corbyn’s office was behind the briefing, telling journalists: “Jeremy won't be getting into personal abuse, as he never has done. He will continue that during this campaign.

"He has said all along he wants there to be a broad, open, democratic participatory campaign, which is respectful for all."

Watch | Owen Smith in 60 seconds

01:03

Asked whether Mr Corbyn was confident he would win the head-to-head contest, the source said: “He is not complacent in anyway, but he was elected with a landslide a year ago.

“By a lot of measures his support has increased among Labour party members and activists and supporters and there is every reason to think he will be re-elected.”

During a string of TV and radio appearances on Wednesday morning, Mr Smith also promised to make Mr Corbyn the Labour Party’s chairman if he wins the leadership contest in an apparent olive branch to his supporters.

Watch | Eagle: I'm confident we can force Corbyn out

00:40

Mr Smith, the former shadow work and pensions secretary, is more than 20 points behind Mr Corbyn with two months to go, according to a YouGov poll this week.

The leadership campaign clash came as a new YouGov poll saw Labour falling to 29 per cent support among voters, 11 points behind the Conservatives.

Watch | Jeremy Corbyn says he would keep submarines patrolling the world without any nuclear weapons